8.5. Fibre Channel Topologies

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Fibre Channel supports the following three topologies:

  • Point to point

  • FC-AL

  • FC-SW

To understand the discussion in this section, you must be familiar with the various Fibre Channel port types as defined in the following table. Refer back to this table as needed during the discussion of the three topologies.

Port

Type

Associated Topology

N_port

Node

Point to point or fabric

NL_port

Node

Node connected to an arbitrated loop

F_port

Fabric

Fabric port

FL_port

Fabric

Fabric connected to an arbitrated loop

L_port

Loop

Hub port on an arbitrated loop

L_C_F

Loop

Link Control Facility (L_C_F) is a hardware facility that attaches to each end of the link and manages transmission and reception of data. It is located within each port, contains a transmitter and receiver, and provides the logical interface to the node.

Rx

 

The receiver (Rx) is the portion of the link control facility dedicated to receiving an encoded bit stream from the media, converting this bit stream into transmission characters, and decoding these characters using the rules specified by FC-PH.

Tx

 

The transmitter (Tx) is the portion of the link control facility dedicated to converting valid data bytes and special codes into transmission characters using transmission code rules. The Tx then converts these characters into a bit stream and transmits the bit stream into the transmission media, which is either optical or electrical.

G_port

 

This is a generic switch port that operates in either E_port mode or F_port mode.

E_port

 

This interswitch expansion port is used to connect to an E_port of another switch to build a larger fabric.


8.5.1 Fibre Channel Arbitrated Loop Topology

By connecting nodes to a Fibre Channel hub, you can create an FC-AL, as shown in Figure 8-16.

Figure 8-16. Fibre Channel arbitrated loop topology.


FC-AL adds capacity (support for up to 126 nodes on a single loop) but not performance. The bandwidth is shared among all active nodes on the loop.

This topology allows a single connection between one pair of ports at any point in time. After a session (two devices communicating) is started, the other devices connected to the loop must wait until the connection ends.

Devices participating in the arbitrated loop share access, but the active link has the full bandwidth. Only one pair of nodes can be communicating on the loop at the same time.

The arbitrated loop topology permits several devices to share the bandwidth of a single loop of fiber running between them. The FC-AL standard is implemented by modifying an N_port to be an NL_port. Each NL_port is attached to one link. The information flows in one direction around the arbitrated loop.

An arbitrated loop is a logical loop and a physical star. In Figure 8-16, X represents the output of one node, and R represents the receiver port of another node. The transmitter port on one node is connected to the receiver port on another node. Notice that information flows in one direction around the loop.

In an arbitrated loop topology, the routing function is distributed to each loop port. The separation of the transmit and receive fibers associated with each port makes this possible.

8.5.2 Switched Fabric Topology

A network of switches in a Fibre Channel environment is referred to as a fabric. Nodes connect into this fabric to access other nodes, as illustrated in Figure 8-17. A wide-open architecture uses intelligent switches to connect many ports.

Figure 8-17. Fibre Channel switched fabric topology.


The Fibre Channel fabric was designed as a generic interface between a node and the physical layer. By adhering to this interface, Fibre Channel nodes can communicate over the fabric with other nodes without knowing about that node.

A fabric is often referred to as a switch topology. Frames are routed through various switches by having the fabric elements interpret the destination address identifier in a frame when it arrives at each fabric element.

Ports on one node can communicate with ports on other nodes connected to the same fabric. With the fabric topology, many connections can be active at the same time.

The any-to-any connection service and peer-to-peer communication service provided by a fabric is fundamental to Fibre Channel architecture. Fibre Channel can support both channel and network protocols simultaneously.

The class of service used influences frame routing. The class of service request is generated in the start of frame delimiter of each frame.

The key advantages of the switch fabric topology include the following:

  • Support for up to 16 million nodes

  • Switches that act like routers (with only those ports communicating seeing the traffic)

  • High-aggregate bandwidth (with multiple paths enabled concurrently)

  • Electrical and logical isolation

  • More expensive than FC-AL

A switched fabric works as follows:

  1. A direct connection to the fabric in the host is through an interface that supports the NL_port protocol.

  2. The switch itself has F_ports to which the cable from another device connects. A whole FC-AL group can attach to a switch through an FL_port.

  3. Most FC-AL groups attach to a switch through their hub; the FL_ports for some switches allow the loop to attach directly without a hub.

8.5.2.1 FABRIC ELEMENT

In the fabric topology, at least one active element (such as a switch, ring, or hub) must be placed between ports. This element is responsible for functions such as responding to fabric login requests, managing class of service for the fabric, and assigning addresses.

A fabric element can

  • Attach N_ports.

  • Attach arbitrated loops.

  • Serve as the root element for distributed fabric elements.

A fabric element is the smallest entity that can function as a complete fabric topology. It must have at least three F_ports or FL_ports to make routing decisions.

When multiple fabric elements are interconnected, they form a cooperative unit that is still a fabric. The term heterogeneous fabric refers to the variations in function that are possible with multiple fabric elements.

8.5.3 Point-to-Point Topology

The simplest Fibre Channel topology is point to point, in which a single link connects only two ports. This topology is inexpensive because no hub is required.

To create larger point-to point configurations, you can provide multiple N_ports on each node.

Each point-to-point connection provides the full bandwidth supported by the N_ports. Depending on the type of link (multimode or single-mode fiber), the two nodes can be separated by up to 500m (multimode fiber) or 10km (single-mode fiber).

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    HP ProLiant Servers AIS. Official Study Guide and Desk Reference
    HP ProLiant Servers AIS: Official Study Guide and Desk Reference
    ISBN: 0131467174
    EAN: 2147483647
    Year: 2004
    Pages: 278

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